This invention relates to a molding apparatus and method for molding a plastic article of varied density and, more particularly, to a molding apparatus and method which allows a plastic article to be molded with a smooth dense outer shell and a foam interior.
Injection molding machines generally include a two section mold unit wherein one of the mold sections is stationary and includes an end gate opening for the injection of mold material into the cavity. The other mold section is generally movable between an open position away from the stationary mold section and a closed position wherein the two mold sections are in sealed contact engagement to form the mold cavity.
Once the mold cavity has been formed a screw or similar injection device is used to inject a plastic material into the mold cavity where the material is cured under pressure. It is well known in the prior art to combine a blowing agent with the plastic material to decrease the density of the finished plastic part.
Injection molding provides an efficient means for producing plastic articles both quickly and economically. It is often desired to create a plastic article of a decreased density which reduces costs by reducing the amount of material required to make the part and significantly decreases the weight of the part which is a highly desirable characteristic for many applications. One way of creating a plastic part of reduced density is through the use of a blowing agent. Most blowing agents are heat activated and when mixed with a plastic material under control conditions they produce bubbles. If the blowing agent is evenly distributed throughout the plastic material the bubbles are generally distributed evenly throughout the plastic and become trapped within the plastic article as the article cures. These trapped bubbles form a cellular structure within the finished article. Although blowing agents typically reduce the cost and weight of finished plastic articles, there are certain drawbacks to the use of blowing agents.
While it is often desirable to have a cellular structure within a plastic article, this cellular structure is typically undesirable on the outer shell of a finished plastic article. Bubbles forming near the outer shell of a curing plastic article often result in unaesthetic holes appearing in the surface of the article or compromise the integrity of the surface by creating sink holes as the article cures or weak points where the article is unable to sustain pressure.
It would be most desirable to mold a plastic part with a uniform cellular structure on the interior which allows the part to be molded lighter and with less material, while at the same time having a denser outer skin which would increase the aesthetic and structural aspects of the finished article. Heretofore, it has been impossible to injection mold plastic material with a blowing agent to produce a plastic article without surface defects.
Injection molding machines generally include a two-section mold unit wherein one of the mold sections is stationary and the other mold section generally is movable between an open position away from the stationary mold section and a closed position wherein the two mold sections are in sealed contact engagement to form a mold cavity. The stationary mold section generally includes an opening for the injection of a mold material into the cavity. While it has been possible in the past to mold the rare interior of a plastic article and then coat this material with a sealer or secondary layer of dense plastic material, such a secondary step is time consuming and costly. It has heretofore not been possible to mold a plastic article with a dense outer shell and a rare interior in a single step.
The difficulties encountered in the prior art discussed hereinabove are substantially eliminated by the present invention.